accept BOWER_FLAGS like NPM_FLAGS and YARN_FLAGS

[fool]

since there is no way to NOT run bower install if there is a /bower.json - we should allow users to pass flags like we do for NPM and YARN

[c0b41]

Bower deprecation soon also yarn support bower installing https://bower.io/blog/2017/how-to-migrate-away-from-bower/

[brycekahle]

Bower is actively encouraging folks to migrate to yarn. I do not think we should add new features for it.

[oskarrough]

For anyone ending up here from a search. Older versions of bower use a registry that might fail on Netlify with a "EINVRES failed 502" error.

If you can't upgrade bower, tell bower to use the new registry with a .bowerrc file. Like this:

{
  "registry": "https://registry.bower.io"
}